To test the hypothesis that the full extent of in vitro cell lysis due to ultrasound becomes evident with time lapse following insonation, human erythrocytes (2% hematocrit) in autologous plasma were mixed with Albunex(R), a pulse echo contrast agent, and exposed to 1-MHz, continuous-wave ultrasound (US) (5 W/cm(2) SPTA intensity) for 60 seconds while in a rotating (200 rpm) dialysis membrane vessel. Exposed and sham-exposed samples were subsequently assayed for hemolysis colorimetrically, either immediately or after a delay of 3 hours. Hemolysis was dependent on the interval between US exposure and assay, with significantly greater lysis evident with delayed assay. There was also temporality in lytic yield with sample number, i.e., with time postpreparation of the blood sample, US-induced cell lysis decreased. The temporality of lytic yield was eliminated by maintenance at ice water temperatures, or by waiting about 1 hour before beginning treatments. The collective data indicate that the full extent of US-induced cell lysis is not evident upon assay immediately after insonation, and that with time postpreparation and preinsonation, erythrocytes may undergo changes in sensitivity to US. (ECHOCARDIOGRAPHY, Volume 13, January 1996)